Mutations in Pohl are responsible for the inherited disorder, the variant form of xeroderma pigmentosum (XP- V). XP-V patients are extremely prone to sunlight induced skin cancers. The high incidence of skin cancers in these patients is due to the absence of functional Pol0, which has the remarkable ability to replicate through UV lesions such as a cis-syn cyclobutane thymine-thymine (T-T) dimer in an error-free way. Unlike classical DNA polymerases that become stalled at the UV induced T-T dimer, Polo can efficiently and accurately replicate past this common sunlight induced lesion. Besides a T-T dimer, Polrl can also efficiently and accurately replicate DNA containing 7,8-dihydro-8-oxoguanine (8-oxoG) adducts formed by oxidative damage. The discovery of Polr I has led to the description of a whole new class of translesion synthesis (TLS) DNA polymerases within the last two years, including the DinB homolog Polk in humans. However, despite similarities in sequence, TLS polymerases differ in the type of DNA damage they bypass, consistent with specialized cellular roles. The specific aims are as follows: 1. Determine the structure of Pobl. 2. Determine the structures of Polrl bound to damaged and undamaged DNA 3. Structure based mutational analysis of Pol'q. 4. Determine the structure of Poll] in complex with PCNA 5. Determine the structure of Dpo4 (a Polk homolog) in the mismatch extension mode These structures will be tested throughout by extensive molecular mechanisms of these new DNA polyrnerases